1. Field of the Invention
The present invention relates to a process and apparatus for producing high purity hydrogen by steam-reforming a carbonaceous fuel. The apparatus comprises a reforming chamber provided with a hydrogen-permeable membrane tube, and an oxidation chamber with an oxidation catalyst bed disposed therein.
2. Description of the Related Art
Steam-reforming of hydrocarbon or carbonaceous fuels, such as methanol, ethanol, gasoline, petroleum and the like, for producing hydrogen for various applications are known in the art. This invention is particularly concerned with steam-reforming of carbonaceous fuels to generate hydrogen for use in fuel cell for electric vehicles and on-site electric power plants.
Steam-reforming of hydrocarbon, such as methanol, is a reversible and endothermic reaction. Thus, heat must be supplied to allow the reaction to proceed and to allow the reaction to reach the equilibrium state. The maximum yield of a reforming reaction can only be reached at the equilibrium state of the reaction. Due to the poor yield at low temperature, the temperature of the steam-reforming reaction often requires the temperature to be raised to 700-900.degree. C. in order to obtain a satisfactory hydrogen yield. Such high reaction temperatures can be reduced upon interrupting the equilibrium state by withdrawing one of the products during the progress of the reaction, while maintaining the same yield.
Traditionally, the purity of hydrogen obtained from the steam-reforming of carbonaceous fuels normally reaches to about 70%, which requires further purification, for example to a level of greater than 95% purity, before application to the fuel cells for technical and economic feasibility.
High purity hydrogen can be obtained by using a thin metal layer of palladium or palladium-alloy membrane as seen in the semiconductor industry. However, such membranes are impractical for application on an industrial scale because of their low hydrogen flux which would result in a demand of greater surface area of the membrane and tremendous expense. Furthermore, the thickness of such thin metal layer is normally too thin to possess sufficient mechanical strength during application, especially under elevated temperatures and pressures.
U.S. Pat. No. 5,451,386 discloses a hydrogen-selective membrane comprising a tubular porous ceramic support having a palladium metal layer deposited on an inside surface of the ceramic support. Using such membrane can provide a hydrogen flux and hydrogen selectivity that is significantly higher than the traditional membranes described above and can possess good mechanical strength for high temperature hydrogen separations, such as when applied in the promotion of ammonia decomposition. While the ceramic supported membrane exhibits a mechanical strength that can be used at high temperatures and pressures, the thickness of the palladium layer of the membrane has to be greater than 10 .mu.m to avoid any defects from taking place. Such limitation would restrict the promotion of hydrogen flux which requires a reduction in thickness of the palladium layer of the membrane.
According to the aforesaid U.S. patent, in Buxbaum et al. it is disclosed that a 2 .mu.m thick of palladium film deposited on a niobium disk fails when applied in the extractions of hydrogen at temperatures above about 500.degree. C. because of the diffusion of niobium into the palladium metal under such high temperatures, and eventually becomes impermeable to hydrogen.
U.S. Pat. No. 5,741,474 discloses a system for producing high-purity hydrogen by reforming a hydrocarbon and/or an oxygen atom-containing hydrocarbon to form a reformed gas containing hydrogen and by separating the hydrogen from the reformed gas. Such system includes a reforming chamber provided with a catalyst for steam-reforming and partial oxidation, and a hydrogen-separating membrane, such as a palladium or palladium-silver alloy membrane. The heat generated by the partial oxidation maybe used as heat to continue the reforming by cooperating with the heat generated by the burning of the non-permeated gas. The burning of the non-permeated gas would, however, require additional fuels to burn the gas. Also, the burning often needs to employ an extremely high temperature, which implies a need of special materials for construction and for prevention of a significant amount of heat loss. Furthermore, the burning of the non-permeated gas in this process is unable to convert completely the gas into a non-polluting gas for discharge into environment.